| Wind speeds and current velocities resolved in east-west and north-south
directions for July 12 to 14, 2000. Isotherms are included to illustrate
thermocline and internal wave activity. High frequency waves cause
smearing of the isotherms when the thermocline is compressed.
Winds flowing to the southwest induced downwelling and flow to the north-east between 3 and 5 m depth. Relaxation of the wind led to upwelling and flow of near-surface currents to the south-west. Currents are reversed between the upper mixed layer and thermocline during upwelling and downwelling resulting in high shear and Richardson's numbers dropping below 0.25. Energy dissipation rates reach 10-7 m2 s-3 at the shear zone and led to entrainment of phytoplankton from the chlorophyll maximum into the upper mixed layer. Current reversals are associated with the high frequency waves in the thermocline (not shown) and in the hypolimnion. Energy dissipation rates are also elevated when high frequency waves are present and when flow reversals occur in the hypolimnion. The currents were measured with a RD Instruments 300 kHz ADCP (acoustic
Doppler current profiler) in Mode 11. This mode allowed 20 cm bin
sizes and resolution of 1 to 2 mm s-1. These current measurements
are the first on this scale from any small lake. Their analyis, in
combination with the concurrent time series measurements of temperature
and temperature-gradient microstructure will allow us to determine key
processes causing turbulence in stratified water bodies.
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